Interpretive Summary: Breakdown of soil organic matter and crop residue can supply a substantial portion of the N that subsequent crops need for optimal growth. The amount of N supplied by soil organic matter and crop residue is difficult to predict. Overestimation of the contribution soil organic matter and crop residue make to meeting the N needs of a crop results in underfertilization which may negatively effect crop yields. Underestimation of the contribution soil organic matter and crop residue make to meeting the N needs of a crop results in overfertilization which increases input costs and potentially contributes to environmental degradation. We used an electronic sensor to measure changes in plant available N early in the growing season as a predictor of the N contribution breakdown of soil organic matter and crop residue would make to the current crop. We found that this approach underestimated the N contribution soil organic matter and crop residue make to crop N needs. Including soil temperature in the calculation to estimate the contribution soil organic matter and crop residue make to crop N needs would likely improve our estimates. We conclude that the sensor used in this study has potential for improving N fertilizer rate recommendations which will improve economic returns to farmers and reduce the potential for environmental degradation.

Technical Abstract:
Nitrogen mineralization contributions to crop N needs are difficult to measure. Methods for accurately estimating N-mineralization early in the growing season would be of great value in optimizing N application rates. Mineralization produces ammonium and nitrate and changes in the soil content of these ions can be measured using electrical conductivity methods. We used electromagnetic induction measurements of apparent electrical conductivity to estimate N-mineralization in unamended control soils, soils receiving inorganic fertilizer, and soils receiving manure and compared these estimates to in situ measured net N mineralization. Estimated N-mineralization was 16% less in control soils, 26% less in inorganic fertilizer soils, and 17% less in manured soils than measured net N mineralized. Underestimation was likely due to temperature effects on mineralization that occurred later in the growing season after apparent electrical conductivity measurements were taken. This approach has potential for generating maps of N-mineralization that could be used for variable rate application of N fertilizer.